Detection and monitoring of airborne substances and analytes in a gas phase have become an important part in public health care, military and customs activities, security surveillance in public buildings and transportation, and in environmental monitoring. Physicians, the police, customs personnel, security personnel and others need detection equipment in order to detect the presence of various substances.
Previous analysis devices for analysing analytes in gas phase use three different absorption schemes. Firstly, the use of an open liquid reservoir with passive particle absorption has been suggested (M. Michalzik, R. Wilke and S. Büttgenbach, Sensors and Actuators B, 2005, 410-415), and (C. Kösslinger, S. Drost, F. Aberl, H. Wolf, S. Koch and P. Woias, Biosensors and Bioelectronics, 1992, 7, 397-404). Secondly, passively controlled surface tension based microfluidic interfaces for airborne sample-to-liquid absorption have been demonstrated (T. Frisk, W. v. d. Wijngaart, D. Rönnholm and G. Stemme, Lab On a Chip, 2006, 6, 1504-1509). Finally, systems with active liquid manipulation can be utilized to capture and transfer airborne particles to microfluidic systems. Desai et al. (A. Desai, S.-W. Lee and Y.-C. Tai, MEMS, 2000, 733-738) demonstrated airborne particle sampling with a liquid meniscus interface with DEP driven particle capture through the air-liquid interface. Recently, Zhao et al. (Y. Zhao and S. K. Cho, Lab On a Chip, 2005, 6, 137-144.) showed particle trapping with EWOD droplet sweeping.
Gast and Fiehn in Lab On a Chip, 2003, 3, 6-10, disclosed a chemical sensor comprising a sieve plate built into a channel system containing a microfluidic system.
The prior art regarding analysis devices for the analysis of analytes in a gas phase include several drawbacks. With open liquid reservoirs, the interfacial area is large but the sensitivity to external influence is large, since the gas liquid interface is easily disturbed. Such systems cannot offer robustness in terms of pressure variation tolerance or invariance to gravitation changes i.e. abrupt movement of the device. Surface tension based passive systems in the prior art can only offer a reduced possibility for a rapid response signal. There may also be problems with achieving an exposed area, which is sufficiently large.
Active liquid manipulation, utilize a moving liquid front, requiring manipulation of particles, the liquid and/or its constituents, with absorption taking place when the liquid reaches particles on the surface. The active liquid manipulation increases the complexity of such devices and negatively affects the potential for portability. Moreover the manufacturing costs increase.
Problems encountered in commercial sensors are long transport paths through tubing, valves, etc, resulting in reagent depletion through parasitic binding and in sample dispersion. Furthermore, difficulties in integration of many stand-alone components hinder development of portable instruments.
One problem in the state of the art regarding analysis devices comprising a quartz crystal microbalance is the sensitivity to external influence and stability.